1. Field of the Invention
This invention relates to an image display apparatus, and particularly to a head mounted type display apparatus or a spectacle type display apparatus which enables an image such as characters or a projected image displayed on display means to be observed as an enlarged virtual image.
2. Related Background Art
With the development of the liquid crystal display element, a head mounted display (HMD) of the so-called see-through type, in which a liquid crystal display element and an observation optical system are provided in front of an observer's eyes, and the observer observes an exterior scene through the optical system and an image is displayed on the liquid crystal display element and simultaneously shown with another image superposed on the exterior scene.
There is also known an image display apparatus in which an image displayed on a display element is changed over according to an observer's visual axis.
The conventional HMD type image display apparatus has attached thereto a mechanism for adjusting the visibility of the optical system relative to the display element, but has suffered from the problem that when the distance of an exterior scene observed by an observer greatly fluctuates, for example, from infinity to several tens of centimeters in front of the observer's eyes, the distance of the virtual image of an image displayed while being superposed on the exterior scene is greatly in discord with the observation distance and a blurred virtual image is visually confirmed.
Also, sometimes the image superposed is not necessary in the whole observation distance and for example, in an observation distance within 2 m, image information is required and in the other observation distances, the image information is not required or becomes harmful.
Also, in an image display apparatus wherein the display image is changed over by the visual axis, it is often the case that the display image is not changed over to an entirely discrete image, but it is desired to move the image being displayed and change over it so as to represent a new portion, whereas the image display apparatus according to the conventional art could not accomplish such a thing.